Materials in which acrylonitrile or methacrylonitrile is polymerized have been widely used because of their excellent heat resistance, weatherability, oil resistance, flame retardancy, etc. Commonly known examples thereof include acrylonitrile-styrene-acrylic rubber copolymer resins (AAS), acrylonitrile-ethylene-styrene copolymer resins (AES), acrylonitrile-styrene copolymer resins (AS), acrylonitrile-butadiene-styrene copolymer resins (ABS), acrylonitrile-chlorinated polyethylene-styrene resins (ACS), acrylonitrile-butadiene copolymer rubber (NBR), and hydrogenated nitrile rubber.
However, all of these materials are formed by random copolymerization of acrylonitrile and other monomers or by graft copolymerization of acrylonitrile onto polymer chains. With respect to block copolymers including polymer blocks containing acrylonitrile and methacrylonitrile as principal constituents, although research has been conducted on the laboratory level, no production has been performed industrially.
In general, in an acrylonitrile-containing polymer, as the acrylonitrile content is increased, oil resistance, wear resistance, heat resistance, and strength are improved, while low-temperature resistance tends to be degraded. In contrast, in a block copolymer in which a polymer block containing acrylonitrile or methacrylonitrile as a principal constituent and a flexible polymer block composed of other constituents, low-temperature resistance is considered to be improved while maintaining excellence in heat resistance, oil resistance, etc.
As the methods for synthesizing block copolymers, living polymerization methods are usually used. Examples of living polymerization methods include living cationic polymerization methods, living anionic polymerization methods, and living radical polymerization methods. Among them, living radical polymerization methods are most useful because of the applicability to a wide variety of monomers and also because of the applicability to water-based polymerization. Commonly known examples of the living radical polymerization methods include a method using nitroxyl radicals, such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radicals, described in Junpo He et al., Polymer, 2000, 41, p. 4573; and an atom transfer radical polymerization method described in Matyjaszewski et al., J. Am. Chem. Soc., 1995, 117, p. 5614. However, in the method using nitroxyl radicals, little has been reported about the polymerization of monomers other than styrene, and no example has been known about the polymerization of acrylonitrile. Furthermore, in general, since the polymerization temperature must be 120° C. or more, this method is not economical for industrial production. On the other hand, in the atom transfer radical polymerization, since nitrile groups coordinate to a metal complex which is a catalyst, polymerization does not proceed satisfactorily. Furthermore, a complicated purification step is required in order to remove the metal complex from the polymer, thus being uneconomical. Additionally, in the living radical polymerization methods described above, it is generally difficult to perform water-based polymerization, such as emulsion polymerization or suspension polymerization. In the living radical polymerization methods described above, there are cases in which it is difficult to perform water-based polymerization, such as emulsion polymerization or suspension polymerization.